1. Field of the Invention
The present invention is directed to wireless communication systems and, more specifically, to the transmission of acknowledgment signals in the uplink of a communication system.
2. Description of the Related Art
A communication system consists of the DownLink (DL), conveying transmissions of signals from a base station (Node B) to User Equipments (UEs), and of the UpLink (UL), conveying transmissions of signals from UEs to the Node B. A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, etc. A Node B is generally a fixed station and may also be referred to as a Base Transceiver System (BTS), an access point, or some other terminology.
The UL signals consist of data signals, carrying the information content, control signals, and Reference Signals (RS), which are also known as pilot signals. The UEs convey UL data signals through a Physical Uplink Shared CHannel (PUSCH). The UL control signals include acknowledgement signals associated with the application of Hybrid Automatic Repeat reQuest (HARQ) and other control signals. A UE transmits an HARQ-ACKnowledgement (HARQ-ACK) signal in response to the reception of Transport Blocks (TBs). Depending on whether the reception of a TB is correct or incorrect, the respective HARQ-ACK bit is an ACK or a NAK which can be respectively represented by a bit value of “1” or a bit value of “0”. The HARQ-ACK signal is transmitted over a Transmission Time Interval (TTI) either in a Physical Uplink Control CHannel (PUCCH) or, together with data, in the PUSCH.
An exemplary structure for the PUCCH transmission in a TTI, which for simplicity is assumed to consist of one sub-frame, is shown in FIG. 1. The sub-frame 110 includes two slots. Each slot 120 includes NsymbUL symbols used for the transmission of HARQ-ACK signals or Reference Signals (RS). Each symbol 130 further includes a Cyclic Prefix (CP) to mitigate interference due to channel propagation effects. The PUCCH transmission in the first slot may be at a different part of the operating BandWidth (BW) than the PUCCH transmission in the second slot. Some symbols in each slot can be used for RS transmission to provide channel estimation and enable coherent demodulation of the received HARQ-ACK signal. The transmission BW is assumed to consist of frequency resource units which will be referred to as Physical Resource Blocks (PRBs). Each PRB is further assumed to consist of NscRB sub-carriers, or Resource Elements (REs), and a UE transmits its HARQ-ACK signals over one PRB 140 in the PUCCH.
An exemplary structure for the HARQ-ACK signal transmission in one of the sub-frame slots is illustrated in FIG. 2. The transmission structure 210 comprises of HARQ-ACK signals and RS to enable coherent demodulation of the HARQ-ACK signals. The HARQ-ACK bits 220 modulate 230 a “Constant Amplitude Zero Auto-Correlation (CAZAC)” sequence 240, for example with Binary Phase Shift Keying (BPSK) for 1 HARQ-ACK bit or with Quadrature Phase Shift Keying (QPSK) for 2 HARQ-ACK bits which is then transmitted after performing the Inverse Fast Fourier Transform (IFFT) operation as it is subsequently described. Each RS 250 is transmitted through the =modulated CAZAC sequence. The signal transmission in FIG. 2 is contiguous in frequency and is referred to as Single-Carrier (SC) transmission.
An example of CAZAC sequences is given by
            c      k        ⁡          (      n      )        =      exp    ⁡          [                                    j            ⁢                                                  ⁢            2            ⁢            π            ⁢                                                  ⁢            k                    Z                ⁢                  (                      n            +                          n              ⁢                                                n                  +                  1                                2                                              )                    ]      where z is the length of the CAZAC sequence, n is the index of an element of the sequence n={0, 1, . . . , Z−1}, and k is the index of the sequence. If z is a prime integer, there are z−1 distinct sequences which are defined as k ranges in {0, 1, . . . , Z−1}. If the PRBs comprise of an even number of REs, such as for example NscRB=12 REs, CAZAC sequences with even length can be directly generated through computer search for sequences satisfying the CAZAC properties.
FIG. 3 shows an exemplary transmitter structure for a CAZAC sequence that can be used without modulation as RS or with BPSK or QPSK modulation as HARQ-ACK signal. The frequency-domain version of a computer generated CAZAC sequence 310 is used. The REs corresponding to the assigned PUCCH BW are selected 320 for mapping 330 the CAZAC sequence, an IFFT is performed 340, and a Cyclic Shift (CS) applies to the output 350 as it is subsequently described. Finally, the cyclic prefix (CP) 360 and filtering 370 are applied to the transmitted signal 380. Zero padding is assumed to be inserted by the reference UE in REs used for the signal transmission by other UEs and in guard REs (not shown). Moreover, for brevity, additional transmitter circuitry such as digital-to-analog converter, analog filters, amplifiers, and transmitter antennas as they are known in the art, are not shown. The reverse (complementary) transmitter functions are performed for the reception of the CAZAC sequence. This is conceptually illustrated in FIG. 4 where the reverse operations of those in FIG. 3 apply. An antenna receives RF analog signal and after further processing units (such as filters, amplifiers, frequency down-converters, and analog-to-digital converters) the digital received signal 410 is filtered 420 and the CP is removed 430. Subsequently, the CS is restored 440, a Fast Fourier Transform (FFT) 450 is applied, and the transmitted REs 460 are selected 465. FIG. 4 also shows the subsequent correlation 470 with the replica 480 of the CAZAC sequence. Finally, the output 490 is obtained which can then be passed to a channel estimation unit, such as a time-frequency interpolator, in case of a RS, or can to detect the transmitted information, in case the CAZAC sequence is modulated by HARQ-ACK bits.
Different CSs of the same CAZAC sequence provide orthogonal CAZAC sequences. Therefore, different CSs of the same CAZAC sequence can be allocated to different UEs in the same PUCCH PRB and achieve orthogonal multiplexing for the respective HARQ-ACK signal transmissions. This principle is illustrated in FIG. 5. In order for the multiple CAZAC sequences 510, 530, 550, 570 generated correspondingly from multiple CSs 520, 540, 560, 580 of the same root CAZAC sequence to be orthogonal, the CS value Δ590 should exceed the channel propagation delay spread D (including a time uncertainty error and filter spillover effects). If Ts is the symbol duration, the number of such CSs └Ts/D┘, where the └ ┘(floor) function rounds a number to its previous integer. Orthogonal multiplexing can also be in the time domain using Orthogonal Covering Codes (OCC). The symbols for HARQ-ACK and RS transmission in each slot are respectively multiplied with a first OCC and a second OCC but further details are omitted for brevity as these multiplexing aspects are not material to the invention. The number of resources in a PUCCH PRB is determined by the product of the number of CS for the CAZAC sequence times the OCC length. For 6 CS, length 4 OCC for the symbols used for HARQ-ACK signal transmission, and length 3 OCC for the symbols used for RS transmission, the number of resources for HARQ-ACK signaling in a PRB is 6×3=18 (the smaller OCC length applies).
A UE can determine the PUCCH resource (PRB, CS, OCC) for its HARQ-ACK signal transmission either through explicit indication from its serving Node B or through implicit indication. The latter can be based on the resources used for the transmission of the Scheduling Assignment (SA) in the Physical Downlink Control CHannel (PDCCH). The SA configures the parameters for the reception by the UE of TBs in response to which the UE subsequently transmits an HARQ-ACK signal. An exemplary PDCCH transmission considers that the REs carrying each SA are grouped into Control Channel Elements (CCEs). For a given number of SA information bits, the number of CCEs depends on the channel coding rate (QPSK modulation is assumed). For a UE with low Signal-to-Interference and Noise Ratio (SINR), the Node B may use a low channel coding rate to achieve a desired BLock Error Rate (BLER) while it may use a high coding rate for a UE with high SINR. Therefore, a SA may require more CCEs for its transmission to a low SINR UE. Typical CCE aggregation levels follow a “tree-based” structure consisting, for example, of 1, 2, 4, and 8 CCEs.
FIG. 6 further illustrates the PDCCH transmission using CCEs. After channel coding and rate matching of the SA information bits (not shown), the encoded SA bits are mapped to CCEs in the logical domain. The first 4 CCEs, CCE1 601, CCE2 602, CCE3 603, and CCE4 604 are used for the SA transmission to UE1. The next 2 CCEs, CCE5 611 and CCE6 612, are used for the SA transmission to UE2. The next 2 CCEs, CCE7 621 and CCE8 622, are used for the SA transmission to UE3. Finally, the last CCE, CCE9 631, is used for the SA transmission to UE4. After further processing which can include bit scrambling, modulation, interleaving, and mapping to REs 640, each SA is transmitted in the PDCCH 650.
At the UE receiver, the reverse operations are performed (not shown for brevity) and if the SA is correctly decoded, the UE proceeds to receive the TBs. A one-to-one mapping exists between the PUCCH resources for HARQ-ACK signal transmission and the CCEs used for the SA transmission. For example, if a single PUCCH resource is used for HARQ-ACK signal transmission, it may correspond to the CCE with the lowest index (first CCE) for the respective SA. Then, UE1, UE2, UE3, and UE4 use respectively PUCCH resource 1, 5, 7, and 9 for their HARQ-ACK signal transmission. If all resources within a PUCCH PRB are used, the resources in the immediately next PRB can be used. The first PUCCH PRB for HARQ-ACK signal transmission may be informed by the serving Node B through broadcast signaling. In order to support higher data rates and improve the spectral efficiency relative to legacy communication systems, BWs larger than the ones of Component Carriers (CCs) for legacy systems are needed. These larger BWs may be achieved by the aggregation of multiple legacy CCs. For example, a BW of 100 MHz may be achieved by aggregating five 20 MHz CCs. The reception of TBs in each DL CC is configured by a respective SA as described in FIG. 6.
Each DL CC is associated with an UL CC which contains respective resources for the HARQ-ACK signal transmission. In case each different DL CC is linked to a different UL CC, the resources for HARQ-ACK signal transmission may be as for the legacy systems. In case multiple DL CCs are linked to the same UL CC for HARQ-ACK signal transmission, separate resources may be pre-assigned in the UL CC for the transmission of HARQ-ACK signals in response to TBs in each of the DL CCs. This is further illustrated in FIG. 7 where two DL CCs, 710 and 720, are linked to one UL CC 730 and the resources for the HARQ-ACK signal transmission in response to TBs transmitted in the first DL CC are always in a first set of UL resources 740 while the resources for the HARQ-ACK transmission in response to TBs transmitted in the second DL CC are always in a second set of UL resources 750.
The conventional approach for a UE to transmit HARQ-ACK signals in response to the reception of TBs in multiple DL CCs is to simply extend the HARQ-ACK signaling method in case of a single DL CC and simultaneously transmit multiple HARQ-ACK signals, each corresponding to a DL CC. The main disadvantage of this approach stems for the limitation in the maximum UE transmission power. Simultaneous transmission of multiple HARQ-ACK signals increases the peak-to-average power ratio (PAPR) of the combined signal transmission as the single-carrier property is not preserved. Also, channel estimation becomes worse as the RS power is distributed in multiple resources and the total interference is increased as HARQ-ACK signals are transmitted in multiple resources.
In order to address the previous shortcomings for the transmission of multiple HARQ-ACK signals, an alternative method is to transmit a single HARQ-ACK signal while selecting the transmission resources to provide additional degrees of freedom and hence allow for more HARQ-ACK information to be implicitly conveyed. For example, if the UE receives a single TB in each of four DL CCs and each DL CC is linked to a different UL CC then, by selecting the UL CC where the HARQ-ACK signal is transmitted, the UE can convey 2 HARQ-ACK bits and convey the remaining 2 HARQ-ACK bits by applying QPSK modulation to the transmitted HARQ-ACK signal. Although this CC selection method avoids the shortcomings of multiple simultaneous HARQ-ACK signal transmissions, it cannot generally provide adequate multiplexing capacity. For example, if the UE receives two TBs in any of the four DL CCs, then at least 5 HARQ-ACK bits will need to be transmitted which is not possible using only UL CC selection and QPSK modulation. Moreover, having a variable UL CC convey the HARQ-ACK signal transmission is not desirable for implementation and performance reasons.
Therefore, there is a need to determine transmission methods for HARQ-ACK signals in response to TBs transmitted in multiple DL CCs that avoid increasing the PAPR and also avoid degrading the reception reliability of the HARQ-ACK signal while providing the required multiplexing capacity for the transmission of the HARQ-ACK bits.
There is another need to minimize the interference generated by the HARQ-ACK signal transmission and minimize the respective required resources by avoiding the transmission of multiple HARQ-ACK signals per UE transmitter antenna.
There is another need to determine rules for applying different principles to the transmission of a single HARQ-ACK signal depending on the number of HARQ-ACK bits that need to be conveyed.